Semiconductor device and method for producing semiconductor device

ABSTRACT

A semiconductor device includes: a first semiconductor layer formed, on a substrate, of a nitride semiconductor; a second semiconductor layer formed, on the first semiconductor layer, of a nitride semiconductor; a source electrode formed on the second semiconductor layer; a drain electrode formed on the second semiconductor layer; a metal oxide film formed, between the source electrode and the drain electrode, on the second semiconductor layer; and a gate electrode formed on the metal oxide film. The metal oxide film includes AlO x  and InO x . AlO x /InO x  in the metal oxide film is greater than or equal to 3.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 15/600,260,filed May 19, 2017, which is based upon and claims the benefit ofpriority of the prior Japanese Patent Application No. 2016-104276, filedon May 25, 2016 the entire contents of which are incorporated herein byreference.

FIELD

The embodiments discussed herein relate to a semiconductor device and amethod for producing a semiconductor device.

BACKGROUND

Materials such as GaN, AlN, and InN that are nitride semiconductors andtheir mixed crystals have a wide band gap and are used for devices suchas high output electronic devices or short wavelength light-emittingdevices. For high output devices, techniques relating to Field-EffectTransistors (FET) and High Electron Mobility Transistors (HEMT) aredeveloped (for example, Patent Document 1). HEMTs using such nitridesemiconductors are used for devices such as high output/high efficiencyamplifiers or high power switching devices.

As for a FET using nitride semiconductors, a HEMT, which uses GaN in anelectron transport layer and uses AlGaN in an electron supply layer, isknown. Two-Dimensional Electron Gas (2DEG) is generated in the electronsupply layer through piezoelectric polarization or spontaneouspolarization in GaN. Further, so as to make output and efficiency of aHEMT higher, a HEMT, which uses GaN in an electron transport layer anduses InAlN in an electron supply layer, is known. Spontaneouspolarization of InAlN is high. Therefore, by using InAlN in the electronsupply layer, it is possible to generate high concentration 2DEG and tocause a drain current to flow more than that of the HEMT using AlGaN inthe electron supply layer.

When InAlN is used in an electron supply layer, the surface of InAlN iseasily oxidized, current collapse is caused by indium oxide (InO_(x))included in oxide of InAlN, and a drain current decreases. BecauseInO_(x) formed by oxidation of InAlN is chemically unstable, an oxygendefect is likely to occur. When an electron is trapped in the oxygendefect in InO_(x), a concentration of the 2DEG decreases, currentcollapse occurs, and a drain current decreases.

RELATED-ART DOCUMENTS Patent Documents

-   [Patent Document 1] Japanese Laid-open Patent Publication No.    2002-359256-   [Patent Document 2] Japanese Laid-open Patent Publication No.    2012-174875-   [Patent Document 3] Japanese Laid-open Patent Publication No.    2013-235986

SUMMARY

According to an aspect of the embodiments, a semiconductor deviceincludes: a first semiconductor layer formed, on a substrate, of anitride semiconductor; a second semiconductor layer formed, on the firstsemiconductor layer, of a nitride semiconductor; a source electrodeformed on the second semiconductor layer; a drain electrode formed onthe second semiconductor layer; a metal oxide film formed, between thesource electrode and the drain electrode, on the second semiconductorlayer; and a gate electrode formed on the metal oxide film. The metaloxide film includes AlO_(x) and InO_(x). AlO_(x)/InO_(x) in the metaloxide film is greater than or equal to 3.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are not restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a structure of a semiconductor devicein which an electron supply layer is formed of InAlN;

FIG. 2 is a diagram illustrating a structure of a semiconductor deviceaccording to a first embodiment;

FIG. 3 is a graph illustrating characteristics, analyzed by XPS, of afilm formed by oxidizing In_(0.18)Al_(0.82) with oxygen;

FIG. 4 is a graph illustrating characteristics, analyzed by XPS, of afilm formed by oxidizing In_(0.18)Al_(0.82) with water vapor;

FIG. 5 is a diagram illustrating a structure of a sample 5A in which afilm oxidized by oxygen is formed on a surface of an electron supplylayer;

FIG. 6 is a diagram illustrating a structure of a sample 6A in which afilm oxidized by water vapor is formed on a surface of an electronsupply layer;

FIG. 7 is a graph illustrating a sheet resistance of the sample 5A and asheet resistance of the sample 6A;

FIG. 8 is a diagram illustrating a structure of a semiconductor deviceused for comparison;

FIG. 9 is a graph illustrating Vds-Id characteristics of thesemiconductor device having the structure illustrated in FIG. 8;

FIG. 10 is a graph illustrating Vds-Id characteristics of thesemiconductor device according to the first embodiment;

FIGS. 11A to 11C are diagrams illustrating processes (1) of a method forproducing the semiconductor device according to the first embodiment;

FIGS. 12A and 12B are diagrams illustrating processes (2) of the methodfor producing the semiconductor device according to the firstembodiment;

FIG. 13 is a diagram illustrating a structure of a semiconductor deviceaccording to a second embodiment;

FIGS. 14A to 14C are diagrams illustrating processes (1) of a method forproducing the semiconductor device according to the second embodiment;

FIGS. 15A to 15C are diagrams illustrating processes (2) of the methodfor producing the semiconductor device according to the secondembodiment;

FIG. 16 is a diagram illustrating a structure of a semiconductor deviceaccording to a third embodiment;

FIGS. 17A to 17C are diagrams illustrating processes (1) of a method forproducing the semiconductor device according to the third embodiment;

FIGS. 18A to 18C are diagrams illustrating processes (2) of the methodfor producing the semiconductor device according to the thirdembodiment;

FIG. 19 is a diagram illustrating a structure of a semiconductor deviceaccording to a fourth embodiment;

FIGS. 20A to 20C are diagrams illustrating processes (1) of a method forproducing the semiconductor device according to the fourth embodiment;

FIGS. 21A to 21C are diagrams illustrating processes (2) of the methodfor producing the semiconductor device according to the fourthembodiment;

FIG. 22 is a diagram illustrating a semiconductor device discretelypackaged according to a fifth embodiment;

FIG. 23 is a circuit diagram of a power supply device according to thefifth embodiment; and

FIG. 24 is a diagram illustrating a structure of a high-output amplifieraccording to the fifth embodiment.

DESCRIPTION OF EMBODIMENT

In the following, embodiments will be described. Note that the samereference numerals are assigned to the same members, and theirdescription may be omitted.

An object in one aspect of the embodiments is to provide, in a HEMTusing InAlN for an electron supply layer, a semiconductor device withwhich a drain current does not decrease.

First Embodiment

First, a decrease of a drain current in a semiconductor device usingInAlN for an electron supply layer will be described with reference toFIG. 1.

As illustrated in FIG. 1, a semiconductor device has a buffer layer 911,an electron transport layer 921, a spacer layer 922, and an electronsupply layer 923, which are stacked on a substrate 910 and formed byepitaxial growth of nitride semiconductors. The substrate 910 is formedof a material such as SiC. The buffer layer 911 is formed of a materialsuch as AlN or AlGaN. The electron transport layer 921 is formed ofi-GaN. The spacer layer 922 is formed of AlN. The electron supply layer923 is formed of InAlN. With this structure, in the electron transportlayer 921, two-Dimensional Electron Gas (2DEG) 921 a is generated in thevicinity of the interface between the electron transport layer 921 andthe spacer layer 922.

A gate electrode 931, a source electrode 932, and a drain electrode 933are formed on the electron supply layer 923. Further, a protective film940 is formed on an area of the electron supply layer 923 on which thegate electrode 931, the source electrode 932, and the drain electrode933 are not formed. The protective film 940 is formed of a material suchas SiN.

In the semiconductor device having such a structure, in a process afterthe electron supply layer 923 is formed and before the gate electrode931 and the protective film 940 are formed, an exposed part of theelectron supply layer 923 is oxidized and thus a metal oxide film 924 isformed. Accordingly, the gate electrode 931 and the protective film 940are formed on the metal oxide film 924 in practice. The metal oxide film924 is a film in which InAlN is oxidized, and includes a large quantityof InO_(x). As described above, because InO_(x) included in the metaloxide film 924 formed by oxidation of InAlN is chemically unstable, anoxygen defect is likely to occur. When an electron 924 a is trapped inan oxygen defect in InO_(x), a concentration of the 2DEG decreases inresponse to this. As a result, current collapse occurs and a draincurrent decreases.

The inventors have examined oxidation of InAlN and found that thecurrent collapse can be inhibited by oxidizing InAlN with water vaporrather than oxidizing InAlN with oxygen. The embodiments are based onknowledge found by the inventors as described above.

(Semiconductor Device)

Next, a semiconductor device according to a first embodiment will bedescribed with reference to FIG. 2.

The semiconductor device according to the first embodiment has a bufferlayer 11, an electron transport layer 21, a spacer layer 22, and anelectron supply layer 23, which are stacked on a substrate 10 and formedby epitaxial growth of nitride semiconductors. The substrate 10 isformed of a material such as SiC. The buffer layer 11 is formed of amaterial such as AlN or GaN. The electron transport layer 21 is formedof i-GaN. The spacer layer 22 is formed of AlN. The electron supplylayer 23 is formed of InAlN. Thus, in the electron transport layer 21,2DEG 21 a is generated in the vicinity of the interface between theelectron transport layer 21 and the spacer layer 22. Note that theelectron supply layer 23 may be a layer formed of InAlGaN. In otherwords, the electron supply layer 23 may be formed of a materialincluding InAlN or InAlGaN. In this application, the electron transportlayer 21 may be referred to as a first semiconductor layer and theelectron supply layer 23 may be referred to as a second semiconductorlayer. The first semiconductor layer may include the buffer layer 11,the electron transport layer 21, and the spacer layer 22.

A source electrode 32 and a drain electrode 33 are formed on theelectron supply layer 23. On a surface in an area of the electron supplylayer 23, where the source electrode 32 and the drain electrode 33 arenot formed, a metal oxide film 24 is formed. The metal oxide film 24 isformed by oxidizing the area of the electron supply layer 23 with watervapor. A gate electrode 31 is formed on the metal oxide film 24. On anarea of the metal oxide film 24, where the gate electrode 31 is notformed, a protective film 40 is formed. The protective film 40 is formedof a material such as SiN. Note that in this application, the protectivefilm 40 may be referred to as an insulation film.

Next, a case of thermally oxidizing InAlN and a case of oxidizing InAlNwith water vapor (steam) will be described. Note thatIn_(0.18)Al_(0.82)N is used as InAlN so as to lattice match with GaN.Both the thermal oxidation and the steam oxidation ofIn_(0.18)Al_(0.82)N are performed for 30 minutes at a temperature of300° C. It is considered that the metal oxide film 924, formed in thesemiconductor device illustrated in FIG. 1, is formed through thermaloxidation because the metal oxide film 924 is formed in a productionprocess after the electron supply layer 923 is deposited (formed). Thatis, it is considered that the metal oxide film 924 is formed byoxidation by oxygen.

FIG. 3 is a result of analyzing, by X-ray Photoelectron Spectroscopy(XPS), the metal oxide film formed by thermally oxidizingIn_(0.18)Al_(0.82). FIG. 4 is a result of analyzing, by XPS, the metaloxide film formed by oxidizing In_(0.18)Al_(0.82) with water vapor. Adetection angle of the XPS in FIG. 3 and FIG. 4 is 15°. Information on asurface layer of a film can be accurately obtained when the detectionangle of the XPS is a low angle rather than a high angle. A value ofAlO_(x)/InO_(x) of the metal oxide film, formed by thermally oxidizingIn_(0.18)Al_(0.82) illustrated in FIG. 3, is 2.3. A value ofAlO_(x)/InO_(x) of the metal oxide film, formed by oxidizingIn_(0.18)Al_(0.82) with water vapor illustrated in FIG. 4, is 10.8.Accordingly, the ratio of AlO_(x) with respect to InO_(x) can beincreased by oxidizing In_(0.18)Al_(0.82) with water vapor in comparisonwith a case of thermally oxidizing In_(0.18)Al_(0.82). For AlO_(x), aninsulation property is high and a defect is less likely to occur incomparison with InO_(x). Therefore, by increasing the ratio of AlO_(x)with respect to InO_(x) in the metal oxide film, it is possible toinhibit current collapse and to inhibit a decrease of the drain current.According to the semiconductor device in the embodiment, because themetal oxide film is formed by oxidizing In_(0.18)Al_(0.82) with watervapor, it is possible to inhibit the current collapse and to inhibitdecreasing of the drain current.

Here, AlO_(x)/InO_(x) indicates a ratio of the number of AlO_(x) withrespect to the number of InO_(x). In other words, AlO_(x)/InO_(x)indicates a ratio of Al atoms with respect to In atoms in a metal oxidefilm. Note that one or more kinds of aluminum oxide and one or morekinds of indium oxide may be included in the metal oxide film.

Next, reaction processes of thermal oxidation and water vapor oxidationof metal will be described. In a case where metal (M) is thermallyoxidized with oxygen (O₂), the metal is directly oxidized by oxygen asindicated in the following formula 1.

4M+30₂→2M₂O₃  <Formula 1>

-   -   (M: Metal)

On the other hand, in a case where metal (M) is oxidized with watervapor (H₂O), after the metal hydroxide is generated, the oxide isgenerated from the metal hydroxide as indicated in the following formula2.

2M+6H₂O→2M(OH)₃+3H₂

2M(OH)₃→M₂O₃+3H₂O  <Formula 2>

-   -   (M: Metal)

Note that in a case where the metal (M) is In, In(OH)_(x) sublimates ata temperature of 150° C. In a case where the metal (M) is Al, Al(OH)_(x)becomes AlO_(x) at a temperature of 300° C. Accordingly, in a case whereInAlN is oxidized with water vapor at a temperature of 300° C.,In(OH)_(x) and Al(OH)_(x) are generated first, but at this temperature,In(OH)_(x) sublimates and Al(OH)_(x) becomes AlO_(x). Therefore, becauseIn becomes In(OH)_(x), sublimates, and decreases in the process of steamoxidation, the ratio of Al with respect to In increases in the metaloxide film formed by oxidizing In_(0.18)Al_(0.82) with water vapor.Thus, it is considered that, when the metal oxide film is formed byoxidizing In_(0.18)Al_(0.82) with water vapor, AlO_(x)/InO_(x) is 10.8,which is high. As described above, in the metal oxide film, as the ratioof AlO_(x) increases, defects decrease and electron traps are reduced.Therefore, decreasing of the drain current can be prevented.

Note that AlO_(x)/InO_(x) in the metal oxide film formed by thermallyoxidizing In_(0.28)Al_(0.82) is 2.3, which is lower than Al/In inIn_(0.18)Al_(0.82)N before oxidized, which is about 4.6. Here, becauseIn is more easily oxidized than the Al included in In_(0.18)Al_(0.82)N,it is estimated that, in a state of not being oxidized sufficiently, aproportion of InO_(x) generated as oxide of In is higher than aproportion of AlO_(x) generated as oxide of Al.

Therefore, according to the embodiment, the value of AlO_(x)/InO_(x) inthe metal oxide film 24 is preferably greater than or equal to 3.Further, the value of AlO_(x)/InO_(x) in the metal oxide film 24 ispreferably greater than or equal to the value of Al/In inIn_(0.18)Al_(0.82)N, and for example, is greater than or equal to 4.6,and especially preferably greater than or equal to 10. In other words,it is preferable that the value of AlO_(x)/InO_(x) in the metal oxidefilm 24 is greater than or equal to the value of Al/In in the electronsupply layer 23. Note that when In_(0.18)Al_(0.82)N is oxidized withwater vapor, because In becomes In(OH)x and sublimates, the value ofAlO_(x)/InO_(x) in the metal oxide film 24 becomes higher than the valueof Al/In in In_(0.18)Al_(0.82)N before being oxidized with water vapor.

Further, according to the embodiment, it is preferable that, when InAlNis oxidized by water vapor (steam), the temperature of the water vaporoxidation is greater than or equal to 300° C. in order to efficientlysublimate In(OH)_(x) generated and to efficiently obtain AlO_(x) fromAl(OH)_(x). Further, the temperature of the water vapor oxidation ispreferably less than or equal to 800° C., and more preferably less thanor equal to 500° C. because In losses occur in the electron supply layer23 when the temperature is excessively high.

Next, a sample 5A, in which an oxidation film is formed by thermallyoxidizing In_(0.18)Al_(0.82)N illustrated in FIG. 5, and a sample 6A, inwhich an oxidation film is formed by oxidizing In_(0.18)Al_(0.82)N withwater vapor illustrated in FIG. 6, are prepared. Then, sheet resistancesin 2DEGs of the respective samples 5A and 6A are measured. FIG. 7illustrates this measured result. The sheet resistances are measured byapplying voltage between the source electrode and the drain electrode.

Note that the sample 5A illustrated in FIG. 5 corresponds to aconfiguration, in which the gate electrode 931 and the protective film940 are not formed in the semiconductor device illustrated in FIG. 1.The sample 6A illustrated in FIG. 6 corresponds to a configuration, inwhich the gate electrode 31 and the protective film 40 are not formed inthe semiconductor device according to the first embodiment illustratedin FIG. 2. Note that in the respective samples 5A and 6A, the electronsupply layers are formed of In_(0.18)Al_(0.82)N, and In_(0.18)Al_(0.82)Nis oxidized for 30 minutes at a temperature of 300° C.

As a result, the sheet resistance of the sample 6A is lower than that ofthe sample 5A as illustrated in FIG. 7. Therefore, electrons trapped ina metal oxide film can be reduced and a decrease in a density of 2DEGcan be inhibited by the configuration having the metal oxide film formedby oxidizing In_(0.18)Al_(0.82)N with water vapor in comparison with theconfiguration having the metal oxide film formed by thermally oxidizingIn_(0.18)Al_(0.82)N. That is, the current collapse can be inhibited andthe decrease of the drain current can be inhibited by the configurationhaving the metal oxide film formed by oxidizing In_(0.18)Al_(0.82)N withwater vapor in comparison with the configuration having the metal oxidefilm formed by thermally oxidizing In_(0.18)Al_(0.82)N.

Next, a semiconductor device illustrated in FIG. 8 having a metal oxidefilm 954 formed by thermal oxidation similar to the semiconductor deviceillustrated in FIG. 1 and the semiconductor device according to thefirst embodiment illustrated in FIG. 2 are prepared to describe a resultof measuring a relationship between Vds (drain-source voltage) and Id(drain current). Note that the metal oxide film 954 of the semiconductordevice having the structure illustrated in FIG. 8 is formed by thermallyoxidizing the surface of In_(0.18)Al_(0.82)N for 30 minutes at atemperature of 300° C. The metal oxide film 24 of the semiconductordevice according to the embodiment illustrated in FIG. 2 is formed byoxidizing the surface of In_(0.18)Al_(0.82)N with water vapor for 30minutes at a temperature of 300° C. FIG. 9 illustrates a relationshipbetween Vds and Id in a case where Vg (gate voltage) is changed in thesemiconductor device having the structure illustrated in FIG. 8. FIG. 10illustrates a relationship between Vds and Id in a case where Vg (gatevoltage) is changed in the semiconductor device according to the firstembodiment illustrated in FIG. 2.

As illustrated in FIG. 9 and FIG. 10, when Vg (gate voltage) is thesame, drain current (Id) flows more in the semiconductor deviceaccording to the first embodiment illustrated in FIG. 2 than in thesemiconductor device having the structure illustrated in FIG. 8. FromFIG. 9 and FIG. 10, on-resistance (Ron) when Vg is 2V in thesemiconductor device having the structure illustrated in FIG. 8 is 3.27Ω·mm, and on-resistance when Vg is 2V in the semiconductor deviceaccording to the first embodiment illustrated in FIG. 2 is 2.65 Ω·mm.Thus, the semiconductor device according to first the embodiment candecrease the on-resistance more than the semiconductor device having thestructure illustrated in FIG. 8.

As described above, in the semiconductor device having the structureillustrated in FIG. 8, the metal oxide film 954, which is formed bythermally oxidizing In_(0.18)Al_(0.82)N, is formed on the surface of theelectron supply layer 923 and electrons are trapped in a large quantityof InO_(x) included in the metal oxide film 954. Thus, a currentcollapse is generated by the electrons trapped in the metal oxide film954, the on-resistance increases, and the drain current decreases. Incontrast, in the semiconductor device according to the first embodimentillustrated in FIG. 2, the metal oxide film 24, which is formed byoxidizing In_(0.18)Al_(0.82)N with water vapor, is formed on the surfaceof the electron supply layer 23. Accordingly, because InO_(x) includedin the metal oxide film 24 is fewer than in the metal oxide film 954formed by thermal oxidation, electrons trapped in the metal oxide film24 are also few. Thus, according to the semiconductor device of theembodiment, current collapse is inhibited, on-resistance is low, and adecrease of the drain current is inhibited.

Further, in the semiconductor device according to the embodiment, themetal oxide film 24 is formed by oxidizing InAlN with water vapor. Thus,in comparison with the metal oxide film formed by thermal oxidation, themetal oxide film 24 according to the embodiment contains a largequantity of AlO_(x), whose insulation property is high. Therefore, agate-leak current can be inhibited.

(Method for Producing Semiconductor Device)

Next, a method for producing the semiconductor device according to thefirst embodiment will be described with reference to FIG. 11 and FIG.12.

First, as illustrated in FIG. 11A, on the substrate 10, the buffer layer11, the electron transport layer 21, the spacer layer 22, and theelectron supply layer 23 are formed by causing nitride semiconductorlayers to epitaxially grow. Thereby, in the electron transport layer 21,the 2DEG 21 a is generated in the vicinity of the interface between theelectron transport layer 21 and the spacer layer 22. The nitridesemiconductor layers are formed by epitaxial growth through MetalOrganic Vapor Phase Epitaxy (MOVPE). Note that these nitridesemiconductor layers may be formed by Molecular Beam Epitaxy (MBE)instead of MOVPE. On the electron supply layer 23, a gap layer, formedof a material such as GaN, may be formed (not illustrated).

For example, a sapphire substrate, a Si substrate, a SiC substrate, or aGaN substrate may be used as the substrate 10. According to theembodiment, a SiC substrate is used as the substrate 10. The bufferlayer 11 is formed of a material such as AlGaN. The electron transportlayer 21 is formed of i-GaN. The spacer layer 22 is formed of AlN. Theelectron supply layer 23 is formed of In_(0.18)Al_(0.82)N.

When these nitride semiconductor layers are deposited (formed) throughMOVPE, trimethyl indium (TMI) is used as a material gas of In, trimethylaluminum (TMA) is used as a material gas of Al, and trimethyl gallium(TMG) is used as a material gas of Ga. NH₃ (ammonia) is used as amaterial gas of N. These material gases are supplied to a reactingfurnace of a MOVPE apparatus using hydrogen (H₂) as a carrier gas.

Subsequently, an element isolation area for isolating an element isformed (not illustrated). Specifically, a photoresist is applied on theelectron supply layer 23, and the photoresist is exposed by an exposureapparatus and developed to form a resist pattern having an opening at anarea where the element isolation area is to be formed. Subsequently,Argon (Ar) ions are injected into the nitride semiconductor layer of thearea, in which the resist pattern is not formed, to form the elementisolation area. The element isolation area may be formed by removing,through dry etching such as Reactive Ion Etching (RIE), a part of thenitride semiconductor layer of the area in which the resist pattern isnot formed. After the element isolation area is formed, the resistpattern is removed by an organic solvent or the like.

Next, as illustrated in FIG. 11B, the source electrode 32 and the drainelectrode 33 are formed on the electron supply layer 23. Specifically, aphotoresist is applied on the electron supply layer 23, and thephotoresist is exposed by the exposure apparatus and developed to form aresist pattern (not illustrated) having an opening at respective areaswhere the source electrode 32 and the drain electrode 33 are to beformed. Subsequently, after stacked metal films, which are formed ofTi/Al, are deposited (formed) by vacuum deposition, the stacked metalfilms that are formed on the resist pattern are immersed in an organicsolvent. Thereby, the stacked metal films are removed together with theresist pattern through lift-off processing. In this way, the remainingstacked metal films form the source electrode 32 and the drain electrode33. Note that the stacked metal films formed of Ti/Al are a Ti film andan Al film that are formed on the electron supply layer 23 in thisorder. Subsequently, a heat treatment is performed at a temperature from400° C. to 800° C. in a nitrogen atmosphere to cause the sourceelectrode 32 and the drain electrode 33 to make an ohmic contact.

Next, as illustrated in FIG. 11C, the exposed surface ofIn_(0.18)Al_(0.82)N forming the electron supply layer 23 is oxidizedwith water vapor to form the metal oxide film 24. Specifically, watervapor at a temperature of from 300° C. to 500° C. is used to oxidizeIn_(0.1)Al_(0.82)N, exposed to the surface, to form the metal oxide film24. At this time, it is preferable to perform the process of water vaporoxidation in a vacuum in order to promote sublimation of In(OH)_(x)generated. The film thickness of the metal oxide film 24 formed byoxidizing In_(0.18)Al_(0.82)N with water vapor as described above isabout 2 nm. Here, In_(0.18)Al_(0.82)N is not very deeply oxidized in thewater vapor oxidation. Therefore, the film thickness of the metal oxidefilm 24, formed by using water vapor at a temperature from 300° C. to500° C. to oxidize In_(0.18)Al_(0.82)N, is less than or equal to 3 nm.Further, if the water vapor oxidation of In_(0.18)Al_(0.82)N isinsufficient, the film thickness of the metal oxide film is thin and aproportion of remaining InO_(x) is large. Therefore, according to theembodiment, it is preferable that the film thickness of the metal oxidefilm 24 is greater than or equal to 1 nm and less than or equal to 3 nm.

Next, as illustrated in FIG. 12A, the protective film 40 having anopening portion 40 a is formed at an area where the gate electrode 31 isto be formed on the metal oxide film 24. Specifically, a SiN film, ofwhich the film thickness is from 10 nm to 100 nm, is deposited (formed)through plasma chemical vapor deposition (CVD) or the like.Subsequently, a photoresist is applied on the SiN film, and thephotoresist is exposed by the exposure apparatus and developed to form aresist pattern (not illustrated) having an opening at an area where thegate electrode 31 is to be formed. Subsequently, the metal oxide film 24is exposed by removing the SiN film, exposed at the opening of theresist pattern, through dry etching such as RIE using fluorine gas asetching gas. In this way, the protective film 40 having the openingportion 40 a is formed at the area where the gate electrode 31 is to beformed. Subsequently, the resist pattern (not illustrated) is removed byan organic solvent or the like. Note that, according to the embodiment,the protective film 40 may be formed of a material such as Al₂O₃, HfO₂,SiO₂, SiON, AlN, or AlON instead of SiN.

Next, as illustrated in FIG. 12B, the gate electrode 31 is formed on themetal oxide film 24 exposed at the opening portion 40 a of theprotective film 40. Specifically, a photoresist is applied on theprotective film 40, the electron supply layer 23, the source electrode32, and the drain electrode 33, and the photoresist is exposed by theexposure apparatus and developed to form a resist pattern (notillustrated) having an opening at an area where the gate electrode 31 isto be formed. Subsequently, after stacked metal films, which are formedof Ni/Au, are deposited (formed) by vacuum deposition, the stacked metalfilms that are formed on the resist pattern are immersed in an organicsolvent. Thereby, the stacked metal films are removed together with theresist pattern through lift-off processing. In this way, the remainingstacked metal films form the gate electrode 31. Note that the stackedmetal films formed of Ni/Au are a Ni film and an Au film that are formedon the electron supply layer 23 in this order.

The semiconductor device according to the first embodiment can beproduced through the above described processes.

Second Embodiment

(Semiconductor Device)

Next, a semiconductor device according to a second embodiment will bedescribed with reference to FIG. 13.

As illustrated in FIG. 13, the semiconductor device according to thesecond embodiment is a semiconductor device having a structure, in whicha metal oxide film 124 is formed. In the metal oxide film 124, a filmthickness of an area located immediately below the gate electrode 31 isthicker than a film thickness of other areas. The metal oxide film 124is formed by a first oxidized area 124 a and a second oxidized area 124b. The first oxidized area 124 a is formed by oxidizing the surface ofthe electron supply layer 23. The second oxidized area 124 b is formedby oxidizing a deeper portion of the electron supply layer 23 than thefirst oxidized area 124 a. A gate-leak current can be further inhibitedby thickening the metal oxide film 124 located immediately below thegate electrode 31.

(Method for Producing Semiconductor Device)

Next, a method for producing a semiconductor device according to thesecond embodiment will be described with reference to FIG. 14 and FIG.15.

First, as illustrated in FIG. 14A, on the substrate 10, the buffer layer11, the electron transport layer 21, the spacer layer 22, and theelectron supply layer 23 are formed by causing nitride semiconductorlayers to epitaxially grow. Thereby, in the electron transport layer 21,the 2DEG 21 a is generated in the vicinity of the interface between theelectron transport layer 21 and the spacer layer 22. Subsequently, theelement isolation area for isolating the element is formed (notillustrated).

Next, as illustrated in FIG. 14B, the source electrode 32 and the drainelectrode 33 are formed on the electron supply layer 23.

Next, as illustrated in FIG. 14C, the exposed surface ofIn_(0.1)Al_(0.82)N forming the electron supply layer 23 is oxidized withwater vapor to form the first oxidized area 124 a. Specifically, watervapor, of which a temperature is greater than or equal to 300° C. andless than 500° C., is used to oxidize In_(0.1)Al_(0.82)N, exposed to thesurface to form the first oxidized area 124 a. At this time, it ispreferable to perform the process of water vapor oxidation in a vacuumin order to promote sublimation of In(OH)_(x) generated. The filmthickness of the first oxidized area 124 a formed by oxidizingIn_(0.18)Al_(0.82)N with water vapor as described above is about 2 nm.

Next, as illustrated in FIG. 15A, the protective film 40, which has theopening portion 40 a at an area where the gate electrode 31 is to beformed, is formed on the first oxidized area 124 a.

Next, as illustrated in FIG. 15B, at the area where the opening portion40 a of the protective film 40 is formed, a deeper portion of theelectron supply layer 23 than the first oxidized area 124 a is oxidizedto form the second oxidized area 124 b. In other words, the secondoxidized area 124 b, which is deeper than the first oxidized area, isformed by oxidizing with water vapor the second semiconductor layer atthe area where the opening portion is formed. The metal oxide film 124is formed by the second oxidized area 124 b and the first oxidized area124 a formed as described above. Specifically, water vapor, of which atemperature is greater than or equal to 500° C. and less than or equalto 800° C., is used to oxidize In_(0.18)Al_(0.82)N, forming the electronsupply layer 23 at the opening portion 40 a of the protective film 40,in order to form the second oxidized area 124 b. At the area where theopening portion 40 a is not formed, the electron supply layer is notoxidized because the protective film 40 has been formed. However, at thearea where the opening portion 40 a is formed, oxidation of the electronsupply layer 23 progresses due to the opening portion 40 a. That is,because the temperature of water vapor is higher than in the case offorming the first oxidized area 124 a, at the area where the openingportion 40 a of the protective film 40 is formed, water vapor entersdeeply through the exposed first oxidized area 124 a. Thus, the deeperportion of the electron supply layer 23 than the first oxidized area 124a is oxidized to form the second oxidized area 124 b. In this way, it ispossible to thicken the film thickness of the metal oxide film 124 atthe area where the opening portion 40 a of the protective film 40 isformed. In the metal oxide film 124 formed as described above, the filmthickness of the area at which the opening portion 40 a of theprotective film 40 is formed is in a range of from 3 nm to 4 nm, and thefilm thickness of other areas is about 2 nm.

Next, as illustrated in FIG. 15C, the gate electrode 31 is formed on themetal oxide film 124 exposed at the opening portion 40 a of theprotective film 40. The gate electrode 31 is formed on the area, wherethe second oxidized area 124 b is formed and the film thickness isthick, of the metal oxide film 124. Thus, the film thickness, locatedimmediately below the gate electrode 31, of the metal oxide film 124 isformed to be thick.

The semiconductor device according to the second embodiment can beproduced through the above described processes.

Note that other configurations of the second embodiment are similar tothose of the first embodiment.

Third Embodiment

(Semiconductor Device)

Next, a semiconductor device according to a third embodiment will bedescribed with reference to FIG. 16.

As illustrated in FIG. 16, the semiconductor device according to thethird embodiment is a semiconductor device having a structure, in whicha gate recess is formed on the electron supply layer 23 and a metaloxide film 224 is formed by oxidizing, with water vapor, the surface ofthe electron supply layer 23 where the gate recess is formed. The gaterecess is formed on the electron supply layer 23 and the gate electrodeis formed on the gate recess so that the gate voltage is made closer tonormally-off.

(Method for Producing Semiconductor Device)

Next, a method for producing the semiconductor device according to thethird embodiment will be described with reference to FIG. 17 and FIG.18.

First, as illustrated in FIG. 17A, on the substrate 10, the buffer layer11, the electron transport layer 21, the spacer layer 22, and theelectron supply layer 23 are formed by causing nitride semiconductorlayers to epitaxially grow. Thereby, in the electron transport layer 21,the 2DEG 21 a is generated in the vicinity of the interface between theelectron transport layer 21 and the spacer layer 22. Subsequently, theelement isolation area for isolating the element is formed (notillustrated).

Next, as illustrated in FIG. 17B, the source electrode 32 and the drainelectrode 33 are formed on the electron supply layer 23.

Next, as illustrated in FIG. 17C, a gate recess 23 a is formed at anarea of the electron supply layer 23 where the gate electrode 31 is tobe formed. Specifically, a photoresist is applied on the electron supplylayer 23, and the photoresist is exposed by the exposure apparatus anddeveloped to form a resist pattern (not illustrated) having an openingportion at an area where the gate recess 23 a is to be formed.Subsequently, a part of the electron supply layer 23 exposed at theopening portion of the resist pattern is removed through dry etchingsuch as RIE to form the gate recess 23 a. In other words, a part of theelectron supply layer 23 is removed to form the gate recess 23 a at thearea on which the gate electrode 31 is to be formed. Subsequently, theresist pattern (not illustrated) is removed by an organic solvent or thelike.

Next, as illustrated in FIG. 18A, exposed In_(0.1)Al_(0.82)N forming theelectron supply layer 23 is oxidized with water vapor to form the metaloxide film 224. Specifically, water vapor at a temperature of from 300°C. to 500° C. is used to oxidize In_(0.18)Al_(0.82)N to form the metaloxide film 224. Thus, the metal oxide film 224 is formed on the surfaceof the electron supply layer 23, and the bottom surface and the sidesurfaces of the gate recess 23 a.

Next, as illustrated in FIG. 18B, the protective film 40 is formed onthe metal oxide film 224. The protective film 40 has an opening portion40 a, at an area where the gate electrode 31 is to be formed, that isthe area where the gate recess 23 a has been formed.

Next, as illustrated in FIG. 18C, the gate electrode 31 is formed on themetal oxide film 224. The gate electrode 31 is formed at the area wherethe gate recess 23 a is formed at the opening portion 40 a of theprotective film 40.

The semiconductor device according to the third embodiment can beproduced through the above described processes.

Note that other configurations of the third embodiment are similar tothose of the first embodiment.

Fourth Embodiment

(Semiconductor Device)

Next, a semiconductor device according to a fourth embodiment will bedescribed with reference to FIG. 19.

As illustrated in FIG. 19, in the semiconductor device according to thefourth embodiment, a gate recess is formed on the electron supply layer23, a metal oxide film 324 is formed by oxidizing, with water vapor, theexposed surface of the electron supply layer 23, and an insulation film340 is formed on the metal oxide film 324. Thus, the metal oxide film324 and the insulation film 340 are formed on the bottom surface and theside surfaces of the gate recess where the gate electrode 31 is to beformed. By forming the metal oxide film 324 and the insulation film 340on the bottom surface and the side surfaces of the gate recess, it ispossible to make the gate voltage closer to normally-off and to furtherinhibit a gate leak-current.

(Method for Producing Semiconductor Device)

Next, a method for producing the semiconductor device according to thefourth embodiment will be described with reference to FIG. 20 and FIG.21.

First, as illustrated in FIG. 20A, on the substrate 10, the buffer layer11, the electron transport layer 21, the spacer layer 22, and theelectron supply layer 23 are formed by causing nitride semiconductorlayers to epitaxially grow. Thereby, in the electron transport layer 21,the 2DEG 21 a is generated in the vicinity of the interface between theelectron transport layer 21 and the spacer layer 22. Subsequently, theelement isolation area for isolating the element is formed (notillustrated).

Next, as illustrated in FIG. 20B, the source electrode 32 and the drainelectrode 33 are formed on the electron supply layer 23.

Next, as illustrated in FIG. 20C, a gate recess 23 b is formed at anarea of the electron supply layer 23 where the gate electrode 31 is tobe formed.

Next, as illustrated in FIG. 21A, In_(0.18)Al_(0.82)N forming theelectron supply layer 23 is oxidized with water vapor to form the metaloxide film 324. Specifically, water vapor at a temperature of from 300°C. to 500° C. is used to oxidize In_(0.18)Al_(0.82)N to form the metaloxide film 324. Thus, the metal oxide film 324 is formed on the surfaceof the electron supply layer 23, and the bottom surface and the sidesurfaces of the gate recess 23 b.

Next, as illustrated in FIG. 21B, the insulation film 340 is formed onthe metal oxide film 324. The insulation film 340 is also formed on themetal oxide film 324 that is formed on the bottom surface and the sidesurfaces of the gate recess 23 b. According to the fourth embodiment, aSiN film, of which the film thickness is from 10 nm to 100 nm, isdeposited (formed) through plasma chemical vapor deposition (CVD) or thelike to form the insulation film 340. The insulation film 340, formed asdescribed above, has a function to decrease a gate-leak current at thearea where the gate electrode 31 is formed and has a function as aprotective film at other areas. Note that, the insulation film 340 maybe formed of a material such as Al₂O₃, HfO₂, SiO₂, SiON, AlN, or AlONinstead of SiN.

Next, as illustrated in FIG. 21C, the gate electrode 31 is formed on theinsulation film 340. The gate electrode 31 is formed at the area wherethe gate recess 23 b had been formed.

The semiconductor device according to the fourth embodiment can beproduced through the above described processes.

Note that other configurations of the fourth embodiment are similar tothose of the first embodiment.

Fifth Embodiment

Next, a fifth embodiment will be described. The fifth embodiment relatesto a semiconductor device, a power supply device, and a high-frequencyamplifier.

The semiconductor device according to the fifth embodiment is asemiconductor device discretely packaged according to one of the firstto fourth embodiments, and the discretely packaged semiconductor devicewill be described with reference to FIG. 22. Note that FIG. 22schematically illustrates the inside of the discretely packagedsemiconductor device in which positions of the electrodes and the likemay be different from those in the first to fourth embodiments. Further,in the description of the fifth embodiment, one transistor having HEMTor UMOS structure may be formed in the semiconductor device according toone of the first to fourth embodiments.

First, a semiconductor device produced according to one of the first tofourth embodiments is cut by dicing or the like to form a semiconductorchip 410, which is a HEMT made of GaN semiconductor materials. Thesemiconductor chip 410 is fixed on a lead frame 420 by a die attachmentagent 430 such as solder. Note that the semiconductor chip 410corresponds to one of the semiconductor devices in the first to fourthembodiments.

Next, a gate electrode 411 is coupled to a gate lead 421 by a bondingwire 431, a source electrode 412 is coupled to a source lead 422 by abonding wire 432, and a drain electrode 413 is coupled to a drain lead423 by a bonding wire 433. Note that the bonding wires 431, 432, and 433are formed of a metal material such as Al. According to the fifthembodiment, the gate electrode 411 is a gate electrode pad in thepresent embodiment, which is coupled to the gate electrode 31 of thesemiconductor device according to one of the first to fourthembodiments. Also, the source electrode 412 is a source electrode pad,which is coupled to the source electrode 32 of the semiconductor deviceaccording to one of the first to fourth embodiments. Also, the drainelectrode 413 is a drain electrode pad, which is coupled to the drainelectrode 33 of the semiconductor device according to one of the firstto fourth embodiments.

Next, resin sealing is performed by a transfer molding method using amold resin 440. In this way, the discretely packaged semiconductordevice such as the HEMT using GaN semiconductor materials can beproduced.

Next, the power source device and the high-frequency amplifier will bedescribed according to the fifth embodiment. The power source device andthe high-frequency amplifier according to the fifth embodiment are apower source device and a high-frequency amplifier using a semiconductordevice according to any one of the first to fourth embodiments.

First, the power source device in the fifth embodiment will be describedwith reference to FIG. 23. The power source device 460 according to thefifth embodiment includes a high-voltage primary circuit 461, alow-voltage secondary circuit 462, and a transformer 463 disposedbetween the primary circuit 461 and the secondary circuit 462. Theprimary circuit 461 includes an AC power supply 464, multiple switchingelements 466 (four in this example of FIG. 23), one switching element467, and the like. The secondary circuit 462 includes multiple switchingelements 468 (three in this example of FIG. 23). In the exampleillustrated in FIG. 23, the semiconductor device according to one of thefirst to fourth embodiments is used as the switching elements 466 and467 of the primary circuit 461. Note that it is preferable that theswitching elements 466 and 467 of the primary circuit 461 are anormally-off semiconductor device. The switching elements 468, used inthe secondary circuit 462, use typical MISFETs (metal insulatorsemiconductor field effect transistor) formed of silicon.

Next, the high-frequency amplifier in the fifth embodiment will bedescribed with reference to FIG. 24. The high frequency amplifier 470 inthe fifth embodiment may be applied to a power amplifier for basestation of a mobile phone, for example. This high-frequency amplifier470 includes a digital predistortion circuit 471, mixers 472, a poweramplifier 473, and a directional coupler 474. The digital predistortioncircuit 471 compensates for non-linear distortion of an input signal.The mixer 472 mixes the input signal compensated for non-lineardistortion, with an alternating current signal. The power amplifier 473amplifies the input signal having been mixed with the alternatingcurrent signal. In the example illustrated in FIG. 24, the poweramplifier 473 includes a semiconductor device according to one of thefirst to fourth embodiments. The directional coupler 474 monitors aninput signal and an output signal. In the circuit illustrated in FIG.24, by turning on/off a switch, for example, it is possible to mix anoutput signal with an alternating current signal by using the mixer 472,and to transmit the mixed signal to the digital predistortion circuit471.

A semiconductor device according to an embodiment includes a firstsemiconductor layer formed, on a substrate, of a nitride semiconductor;a second semiconductor layer formed, on the first semiconductor layer,of a nitride semiconductor; a source electrode formed on the secondsemiconductor layer; a drain electrode formed on the secondsemiconductor layer; a metal oxide film formed, between the sourceelectrode and the drain electrode, on the second semiconductor layer;and a gate electrode formed on the metal oxide film, wherein the secondsemiconductor layer is formed of a material including InAlN or InAlGaN,and wherein a value of AlO_(x)/InO_(x) in the metal oxide film isgreater than a value of Al/In in the second semiconductor layer.

The embodiments have been specifically described above, but the presentinvention is not limited to the specific embodiments and variousmodifications and variations may be made without departing from thescope of the present invention.

All examples and conditional language provided herein are intended forpedagogical purposes of aiding the reader in understanding the inventionand the concepts contributed by the inventors to further the art, andare not to be construed as limitation to such specifically recitedexamples and conditions, nor does the organization of such examples inthe specification relate to a showing of superiority and inferiority ofthe invention. Although one or more embodiments of the present inventionhave been described in detail, it should be understood that variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A method for producing a semiconductor device,the method comprising: forming, on a substrate, a first semiconductorlayer of a nitride semiconductor; forming, on the first semiconductorlayer, a second semiconductor layer of a nitride semiconductor; forminga source electrode and a drain electrode on the second semiconductorlayer; forming a metal oxide film by oxidizing, with water vapor, asurface of the second semiconductor layer between the source electrodeand the drain electrode; and forming a gate electrode on the metal oxidefilm.
 2. The method according to claim 1, wherein the metal oxide filmis formed by oxidizing the surface of the second semiconductor layerwith water vapor of which a temperature is greater than or equal to 300°C. and less than or equal to 800° C.
 3. The method according to claim 1,wherein the metal oxide film is formed by oxidizing the surface of thesecond semiconductor layer with water vapor of which a temperature isgreater than or equal to 300° C. and less than or equal to 500° C. 4.The method according to claim 1 further comprising: forming aninsulation film, having an opening portion at an area where the gateelectrode is to be formed on the metal oxide film, after the metal oxidefilm is formed, wherein the gate electrode is formed on the metal oxidefilm at the opening portion of the insulation film.
 5. The methodaccording to claim 1 further comprising: removing a part of the secondsemiconductor layer to form a gate recess at an area on which the gateelectrode is to be formed, after the second semiconductor layer isformed, wherein the metal oxide film is formed by oxidizing the secondsemiconductor layer with water vapor after the gate recess is formed,and wherein the gate electrode is formed on the area where the gaterecess is formed.
 6. A method for producing a semiconductor device, themethod comprising: forming, on a substrate, a first semiconductor layerof a nitride semiconductor; forming, on the first semiconductor layer, asecond semiconductor layer of a nitride semiconductor; forming a sourceelectrode and a drain electrode on the second semiconductor layer;forming a first oxidized area by oxidizing, with water vapor, a surfaceof the second semiconductor layer between the source electrode and thedrain electrode; forming, on the first oxidized area, an insulation filmhaving an opening portion at an area where the gate electrode is to beformed; forming, by oxidizing with water vapor the second semiconductorlayer at the area where the opening portion is formed, a second oxidizedarea that is deeper than the first oxidized area, the first oxidizedarea and the second oxidized area forming a metal oxide film; andforming a gate electrode on the metal oxide film at the opening portionof the insulation film.
 7. The method according to claim 6, wherein,when the first oxidized area is formed, a temperature of the water vaporis greater than or equal to 300° C. and less than or equal to 500° C.,and wherein, when the second oxidized area is formed, a temperature ofthe water vapor is greater than 500° C. and less than or equal to 800°C.
 8. The method according to claim 1, wherein the second semiconductorlayer is formed of a material including InAlN or InAlGaN.